1. Field of the Invention
The present invention relates to an optical coupler in which a light signal transmitted through one of the optical fibers is divided and transmitted through both of the optical fibers.
2. Description of the Related Art
In general, an optical fiber comprises a cylindrical core with a high index of refraction, a cylindrical cladding formed around and covering the core and having a low index of refraction, a cylindrical first covering formed around and covering the cladding, and a cylindrical second covering formed around and covering the first covering. Two such optical fibers are prepared. When the cladding of each optical fiber is formed into a proper length with a taper of a proper thickness, and the claddings are joined together, the light signal transmitted through one of the optical fibers can be divided and transmitted through both of the optical fibers at the portion where the optical fibers are joined together. In such an optical coupler, however, a desired dividing characteristic can not be obtained when the length and thickness of the cladding joined portion change.
A conventional example of such an optical coupler is disclosed in Japanese Unexamined Patent Publication No. 6-308354 which proposes a method of affixing to a retainer member the claddings, the first coverings, and the second coverings. In the method, predetermined lengths of the first and second coverings of two optical fibers are stripped and exposed. Then, the portion where the claddings are joined together is inserted into the retainer member in order to bond the claddings, the first coverings, and the second coverings of the two optical fibers to both sides of the retainer member, using an adhesive.
In the aforementioned optical coupler, however, the cladding, the first covering, and the second covering of each optical fiber are integrally affixed to the retainer member, so that when an external mechanical stress is exerted onto the second coverings, or when a stress causes expansion or contraction of the second coverings caused by changes in environmental temperatures, the stress acts directly on the cladding joined portion. This leads to, for example, deterioration of the optical properties of the optical coupler as a result of changes in the length of the cladding joined portion, or in the worst case to breakage of the cladding joined portion. In addition, the adhesive, used as an affixing means for affixing the optical fibers, needs to have high adhesive strength with respect to both the optical fibers and the retainer member, taking into account the use environment of the adhesive which expands or contracts due to changes in temperature and humidity. Therefore, the types of materials which may be used for forming the conventional optical coupler are limited in number.